RF excited, sealed-off, diffusion cooled CO2 lasers operate at relatively low pressures, such as pressures less than 150 Torr, with 50-100 Torr being typical. CO2 lasers that operate at higher gas pressure can generate output pulses with faster rise and fall times, as well as with shorter pulse widths, than CO2 lasers that operate at lower gas pressures. In addition, higher pressure CO2 lasers are capable of emitting higher peak power pulses. Faster rise and fall times, shorter pulse widths and higher peak power laser pulses are desirable for many material processing applications, such as hole drilling.
Generally, the gain line-width of CO2 lasers increases by approximately 5 MHz per Torr of pressure under normal excitation above threshold conditions. This increased line-width with increased pressure provides some frequency tunability. For example, at 500 Torr, the gain line width of CO2 lasers is approximately 2.5 GHz. While this gain band-width is modest by solid-state laser standards, it is still useful in tuning the CO2 laser to some absorption lines of selected gases and vapors of interest, such as in remote sensing applications and in pumping molecular lasers operating in the THz region of the spectrum.
Due to the very narrow pumping lines of the heavy molecules used in a THz laser, even a limited frequency tuning capability enables one to adjust the pumping wave-length of the CO2 laser to the peak of the absorption line of the molecular laser, thereby increasing pumping efficiencies and laser output power. This tunability also increases the number of THz laser lines by enabling THz laser operation with molecules that presently cannot be pumped with CO2 lasers.
Unfortunately, it is difficult to obtain CO2 discharges having a uniform “glow,” without arcs or hot spots in the discharge, as the gas pressure is increased above approximately 150 Torr. In addition, the large percentage of He making up the CO2 laser gas mixture, such as a ratio of six parts He to one part each of CO2 and N2, makes it easy to experience the generation of corona ionization of the gas around the hot electrode, as well as the inductors contained within the gas plenum chamber that contains the discharge along with the laser resonator. The inductors are used to equalize the RF electric field along the length of the electrode when the RF wavelength is an appreciable fraction of the electrode length. Under present state of the art technologies, the existence of “corona” is difficult to prevent due to the high RF electric fields required to generate discharges at the higher gas pressures. These difficulties have prevented the reliable operation of RF excited, sealed-off, diffusion cooled CO2 lasers above about 150 Torr.
The existence of corona ionization within the gas plenum chamber of the laser, as well as the presence of arcs or hot spots, can reduce the efficiency of the laser and, if significantly severe, prevent laser oscillation. Arcs in the laser discharge also contribute to poor beam quality, as well as to amplitude variations in laser output over time. Reduction in the efficiency of the laser occurs because the arcs tend to heat the laser gas, thereby reducing the over-population between the upper and lower laser levels of the CO2 molecules. Reduction in laser efficiency from the existence of a corona occurs because the flow of current from the corona flows to the grounded metal container housing the gas plenum. This undesired current flow from the corona occurs outside of the discharge that generates optical gain, thereby representing a power loss. Grounding of the metal laser housing is desirable in order to minimize stray RF emission from the laser.
Some of the CO2 pulsed laser material processing needs are being addressed by Q-switched CO2 waveguide lasers. These lasers can provide up to 80 W of average power, with peak powers around 800 times the average power (approximately 65 kW peak powers). The laser also can provide super pulsed RF excitation of the discharge at a 30-50% duty cycle, with pulse repetition frequencies up to 150 KHz with continuous wave RF pumping. Pulse energies of 0.1-6 mJ can be obtained, with pulse widths of 70-200 nsec and a pulse rise time of approximately 50 nsec at approximately 100-150 Torr gas pressure. The relatively low pulse energies, coupled with the relatively higher cost associated with the need for an electro-optical (EO) crystal (usually CdTe), a polarizer, and fast electronics for switching, prevent Q-switched CO2 lasers from serving many of the needs of the material processing industry.
An alternate pulse laser technology that serves an even smaller portion of the material processing industry than Q-switched lasers includes Transverse Excited Atmospheric (TEA) lasers. TEA lasers address a smaller portion of the needs of the material processing industry because TEA lasers do not have sealed off operation, long operating life times (due to sputtering of the electrode contained within the discharge that is caused by the dc discharge), or high pulse repetition frequencies, as less than 300 Hz is typical.
Most pulsed laser material processing applications are being served by super pulsed CO2 wave-guide and slab lasers, with emphasis on slab laser technology. The lower pressure (typically less than 150 Torr) utilized in these lasers yields relatively long laser pulse rise and fall times (typically 30-50 microsec), as well as long pulse widths (typically greater than 30 microseconds) and a relatively high duty cycle (typically around 30-50%). These performance parameters limit the use of super pulsed wave-guide and slab lasers for some of the material processing applications that require faster rise and fall times, as well as higher peak power pulses.
To obtain the faster rise and fall times that are required for obtaining the desired superior hole qualities drilled in printed circuit board (PCB) materials, using sealed-off, low pressure, RF excited CO2 slab lasers, manufacturers of such CO2 laser hole drilling systems utilize optical switches to sharpen the rise and fall times of the laser pulses, such as is described in U.S. Pat. No. 6,826,204, filed Nov. 30, 2004, entitled “Apparatus For Modifying CO2 Slab Laser Pulses,” which is hereby incorporated herein by reference. Acousto-optical or electro-optical switches can be used to perform the pulse sharpening. This brute force approach adds considerable cost to these systems due to the addition of the optical switches and their associated electronics, as well as the necessity of using a higher output power laser to compensate for the laser energy thrown away by clipping the front and back ends of the laser pulses. The upper pulse repetition frequency required in these PCB hole drilling systems is determined by the present speed limitation associated with scanning mirror galvanometer technology, which is approximately 3 to 4 KHz at present.
Thus, there is a need in the laser material processing industry for a pulsed CO2 laser that is sealed-off, has a long operating life-time and is capable of operation at high gas pressures (e.g., between 150 Torr and atmosphere) to obtain relatively fast rise and fall time pulses (less than 1 microsec). There also is a need for such a laser to emit relatively short pulse widths (5-20 microsec), with pulse energies up to and exceeding 30 mJ and having reasonably high pulse repetition rates (such as up to 10 KHz). It also is desirable for these lasers to have a low pulsed RF power duty cycle (such as 10-20%), thereby enabling the generation of high peak power pulses, as well as reasonable average power (such as approximately 50 W or more) and reasonably high peak powers (such as up to 3KW). Realizing a laser with these characteristics will satisfy laser processing applications that are not presently satisfied by the existing CO2 laser technologies. Further, there is a need for tunability with the use of CO2 lasers in remote sensing of gases and vapors applications, as well as in pumping THz molecular lasers.